Track ascii-ness of data in externalized strings.

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 1e4d4a4c14d8550301f5ff09b9ed6a3b0c9e6fff..560db37a8708b1750b6edbd55efc7959a798f5e4 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -678,6 +678,9 @@ Object* String::SlowTryFlatten(PretenureFlag pretenure) {
 
 
 bool String::MakeExternal(v8::String::ExternalStringResource* resource) {
+  // Externalizing twice leaks the external resouce, so it's
+  // prohibited by the API.
+  ASSERT(!this->IsExternalString());
 #ifdef DEBUG
   if (FLAG_enable_slow_asserts) {
     // Assert that the resource and the string are equivalent.
@@ -697,13 +700,16 @@ bool String::MakeExternal(v8::String::ExternalStringResource* resource) {
     return false;
   }
   ASSERT(size >= ExternalString::kSize);
+  bool is_ascii = this->IsAsciiRepresentation();
   bool is_symbol = this->IsSymbol();
   int length = this->length();
   int hash_field = this->hash_field();
 
   // Morph the object to an external string by adjusting the map and
   // reinitializing the fields.
-  this->set_map(Heap::external_string_map());
+  this->set_map(is_ascii ?
+                Heap::external_string_with_ascii_data_map() :
+                Heap::external_string_map());
   ExternalTwoByteString* self = ExternalTwoByteString::cast(this);
   self->set_length(length);
   self->set_hash_field(hash_field);
@@ -713,7 +719,9 @@ bool String::MakeExternal(v8::String::ExternalStringResource* resource) {
   if (is_symbol) {
     self->Hash();  // Force regeneration of the hash value.
     // Now morph this external string into a external symbol.
-    this->set_map(Heap::external_symbol_map());
+    this->set_map(is_ascii ?
+                  Heap::external_symbol_with_ascii_data_map() :
+                  Heap::external_symbol_map());
   }
 
   // Fill the remainder of the string with dead wood.